1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic system.
2. Description of the Related Art
In an image forming apparatus of an electrophotographic system, a laser beam is emitted on a rotating photosensitive drum to expose the surface of the photosensitive drum to form an electrostatic latent image. It is known that a position deviation occurs between scans when a toner is developed for high-speed printing, due to an exposing position variation of a scan interval relative to the rotation direction or due to a rotation variation of the photosensitive drum.
The position deviation degrades the image quality or reduces the lifetime due to nonconformity of charge bias timing provided to an image processing member in image formation or due to nonconformity of rotation start timing.
In Japanese Patent Publication No. S63-055708, an electrostatic latent image depicted on a photosensitive drum and a surface potential of the photosensitive drum changed by the electrostatic latent image are detected, and the strength of the laser beam is controlled based on the detected potential signal. This can prevent the degradation in the image quality caused by the relative position deviation in the vertical scanning direction periodically generated between the laser beam and the photosensitive drum.
In Japanese Patent Application Laid-Open No. H06-274077, a gear that rotates a photosensitive drum rotates an integer number of times while the photosensitive drum rotates from an exposing position to a transfer position to prevent expansion and contraction of a toner image in the rotation direction due to rotation unevenness of the photosensitive drum. Equalization of the rotation unevenness of the photosensitive drum between the exposing position and the transfer position is proposed.